It has become desirable to employ non-impact xerographic-type printers for text and graphics. In such a printer, an electrostatic charge is formed on a photoreceptive surface of a moving drum or belt, and selected areas of the surface are discharged by exposure to light. A printing toner is applied to the drum and adheres to the areas having an electrostatic charge and does not adhere to the discharged areas. The toner is then transferred to a sheet of plain paper and is heat-fused to the paper. By controlling the areas illuminated and the areas not illuminated, characters, lines and other images may be produced on the paper.
One type of non-impact printer employs an array of light emitting diodes (commonly referred to herein as LEDs) for exposing the photoreceptor surface. A row, or two closely spaced rows, of minute LEDs are positioned near an elongated lens so that their images are arrayed across the surface to be illuminated. As the surface moves past the line of LEDs, they are selectively activated to either emit light or not, thereby exposing or not exposing, the photoreceptive surface in a pattern corresponding to the LEDs activated.
To form good images in an LED printer, it is desirable that all of the light emitting diodes produce the same light output when activated. This assures a uniform quality image all the way across a paper. The light output from an LED depends on a number of factors including current, temperature, and processing parameters for forming the LED which may affect its light output as a function of current.
Light emitting diodes for print heads are formed on wafers of gallium arsenide or the like, suitably doped to conduct current and emit light. Long arrays of LEDs are formed on a wafer which is cut into separated dice, each having an array of LEDs. A row of such dice are assembled end-to-end to form a print head array. The light output of the LEDs on a given die are usually reasonably uniform, however, there may be variations from die to die as processing parameters differ between dice. There is some variation within dice from an individual wafer and greater variation from wafer to wafer.
The LEDs are driven by power supplies on integrated circuit chips. The current output of these chips may also vary depending on processing parameters in making these chips. Such variations may compound the variations in the LED dice.
A parameter that is partly LED power supply dependent is the rise time for current flow. This is significant, since the exposure of the photoreceptive surface is a function of both intensity and illumination time. In an LED print head, there may be a few thousand LEDs across the width of the photoreceptive surface. The current in each LED may also be affected by the number of LEDs enabled at any time. Thus, there may be a relatively high current and concomitant higher light intensity or total exposure when a few LEDs are enabled, as compared with the current and light output when a very large number of LEDs are enabled.
It is desirable, therefore, to provide a power supply for an array of LEDs which assures uniform light output across the array and a light output substantially independent of differences in the number of LEDs enabled.